Brainstem chemosensors provide the normal drive to breathe, thereby setting the minute-to-minute pulmonary alveolar ventilation. According to our best evidence, the primary stimulus to these chemosensors is H ion in brain extracellular fluid. The intensity of this central chemical stimulus to breathe is not completely subject to changes in blood plasma H ion concentration, being set by regulated acid-base transport at the blood-brain barrier. To understand how the primary stimulus to breathing is determined, we seek: 1) to determine the mechanism of brain extracellular fluid acid-base control, and 2) to determine to what degree each of the different blood-brain barrier epithelia participate in this control. Toward these ends, we are studying cerebrospinal fluid acid-base regulation in a mammalian isolated spinal cord preparation. Studies underway, which will continue through the next year, involve replacement of native spinal fluid with normal or acidic artificial spinal fluid. We will study the effects of a variety of specfic epithelial transport inhibitors on: 1) the response with normal fluid to changes in plasma acid-base status, and 2) the rate of return to normal following acidification of the spinal fluid compartment. Our working hypothesis of an anatomically-reversed renal proximal tubule-like transport system will be supported if we can show that blocking the HCO3 minus exit from the membrane facing spinal fluid with agents such as SITS, DIDS, probenecid or perchlorate impairs regulation and will be supported if we can show an inhibitory effect on the rate of (HCO3) recovery of carbonic anhydrase inhibition with methazolamide. At the same time, we will further characterize the acid-base regulating capability of the blood-brain barrier epithelia involved in this preparation, the spinal cord parenchymal capillary endothelium and the arachnoid membrane.